Looking at the subscriber base this way is incorrect. $40 spent on an expansion pack should be based outside of the recurring subscription. WoW and FFXI are very good examples of how this is true.
WoW is $15/month (Month-2-Month) + $40 for a New expansion = $70
FFXI is $15/month (Month-2-Month with additional slots for 3 Characters) + $30 for a New expansion = $60
If you're basing it off the 2 months of service plus expansion, SWTOR is still a steal. Keep in mind that WoW only gave their subscribers 5 additional levels for a $40-$50 expansion pack when brand new and waiting a couple years. FFXI held their cap at 75 for 2 out of the 3 expansions before lifting their cap to 99 also charging roughly $30-$40.
SWTOR not only lifted the cap in a year of service, but also gave expanded content.
Like others have stated, if you feel your $15 isn't being well used - don't pay. Preferred Status isn't horrible, but it isn't as beneficial as Subscriber Status. I like not worrying about a delay in chat, queuing for PvP whenever I feel like it, wearing purple armor as I see fit, running Operations often on all my alternate characters, and having a ton of character slots to just get a full experience of the game.
Now if only I felt more of my $15 was going to fixing bugs and existing issues .............